The present invention relates to a method for recovering a hydroxyaromatic compound from aqueous extract streams. In particular, this invention relates to methods for recovering a hydroxyaromatic compound from aqueous extract streams generated in the production of diaryl carbonates.
Diaryl carbonates, and diphenyl carbonate in particular, are valuable monomer precursors for the preparation of polycarbonates by melt transesterification. An advantageous route for the synthesis of diaryl carbonates is the direct carbonylation of hydroxyaromatic compounds by carbon monoxide and an oxidant in the presence of a catalyst.
A wide range of catalysts may be used in this preparation of diaryl carbonates. For example, U.S. Pat. No. 4,187,242 to Chalk discloses catalysts derived from Group VIIIB metals, i.e., metals selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or complexes thereof. U.S. Pat. Nos. 5,231,210 to Joyce, et al., 5,284,964 and 5,760,272 to Pressman et al., and 5,399,734 to King, Jr., et al. further disclose the use of co-catalysts, including metal co-catalyst species such as cobalt pentadentate complexes and complexes of cobalt with pyridines, bipyridines, terpyridines, quinolines, isoquinolines, aliphatic polyamines such as ethylenediamine, crown ethers, aromatic or aliphatic amine ethers such as cryptands, and Schiff bases, in combination with organic co-catalysts such as terpyridines and quaternary ammonium or phosphonium halides. In U.S. Pat. No. 5,498,789 to Takagi et al., the catalyst system consists of a palladium compound, at least one lead compound, at least one halide selected from quaternary ammonium halides and quaternary phosphonium halides, and optionally at least one copper compound.
As can be seen from the above brief review, the crude reaction mixtures arising from the production of diaryl carbonates can contain complex mixtures of catalyst and co-catalyst metals, and organic products and by-products. The cost of commercially implementing direct oxidative carbonylation is heavily dependent on a combination of the efficiency of the catalyst package and on the ability to reclaim and recycle the expensive catalyst components and unconverted hydroxyaromatic starting material, in particular phenol.
It has been found that aqueous solvent extraction of crude diphenyl carbonate mixtures gives rise to an aqueous extract stream containing from about 0.5 to about 20% phenol, and between about 80% to about 99% water. The aqueous stream may further comprise metals, water-soluble organic materials, including other hydroxyaromatic compounds in addition to phenol, and any extractants (e.g., salts, acids, or complexing agents) added to the original aqueous extract. The presence of phenol in these aqueous extract streams may interfere with subsequent manipulations of the stream, for example by impeding phase separation. The presence of phenol starting material and other hydroxyaromatic compounds formed as by-products or present as impurities further complicates disposal of the stream. Methods for the recovery of a hydroxyaromatic compound from such aqueous extract streams would therefore be both financially and environmentally desirable.